Dear SPM'ers
I would be most grateful for any advice or comments. (Sorry to repeat
myself,
but my previous mail (29th Sept.) didn't seem to elict any responses.)
We're using [15O]H2O- PET to study effects of deep anesthesia on rCBF.
The problem is that the results of SPM subtraction analysis (anesthesia vs.
awake) show large clusters of "relatively increased flow" situated in white
matter. Clusters are partly masked out by Analysis treshold (0.8).
Quite frankly, I'm not sure how I should feel about such results. In certain
sense they are logical as anesthesia does reduce flow more in gray than in
white matter. [A separate ROI analysis on quantitative images showed that
regional flow at awake was roughly 65 (ml/100g/min) in gray matter and 25 in
white matter. During anesthesia flow values were reduced to 29 and 14,
respectively. Thus, the gray/white-matter flow ratio reduced from 2.6 to 1.8
(30% reduction).]
My main concern is that this kind of major "shift" might affect the validity
of global flow scaling and therefore create a see-saw effect that
compromises all the statistics. Any comments? We used mean voxel value (w.
fullmean/8 mask) for global flow calculation, and proportional scaling for
global normalisation.
I couldn't find anything quite similar from the e-mail archive. I would
highly appreciate any suggestions - or questions.
Yours,
Kaike
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